Furocoumarins (psoralens) are skin-erythemal and carcinogenic sensitizers under the influence of near UV and UV light. Their photodynamic action is apparently associated with their photochemical reactions with nucleic acids and other cellular components. In the present proposal, modifications of DNA and tRNA by the excited state of psoralens will be further studied with a special emphasis to characterize the cross-linking and single-strand monoaddition of psoralens to DNA. The effects of the photomodification of nucleic acids on replication, transcription and translation processes will be examined in order to ascertain the photobiological consequences of the interactions between nucleic acids and psoralens.